text:
Is a VFR flight following request implied when talking to tower and advising them of your intended airport?
If I am at a towered airport in class delta airspace and I'm departing VFR and tell the ground where I want to go, is it implied to the controller that I'm requesting VFR flight following?
Let's say I'm at the Chandler Municipal airport (KCHD), and I'm headed to the Scottsdale airport, which is in another class delta, and I probably need to transit the Phoenix bravo airspace.
I call Chandler Tower up and say:
"Skyhawk 283JS with information sierra, FBO, north departure to Scottsdale airport"
Will the tower controller coordinate flight following for me and assign a squawk based on this request? Or do I need to specifically request "VFR flight following" if I want it?

It is not. You must explicitly add

Requesting VFR Flight Following

Depending on the airport in question the tower may simply tell you to contact approach once in the air to request flight following. Other towers may set it up for you.
The notation of your destination is extraneous. When departing you generally tell the tower you direction of departure not your destination.
Flight flowing is also not an implied bravo clearance, you will need to request that once in the air. If you are taking off from a towered field you can ask for a handoff when requesting your clearance, this may help facilitate your clearance request as well as your flight following request.

For me (class C controller) if a pilot on the ground explicitly mentions their destination airport I query to confirm they want flight following. They do more often than not. I would imagine this is correlated even stronger at class D airports where VFR departures are normally not given squawk codes unless requesting flight following.

The VFR flight following is not implied even if the controller assigns you a squawk code. You have to explicitly ask for it as Dave mentioned in his answer above.

Got it. Any idea on how the controller remembers which squawking flights have flight following, and which do not?

No, I don't. I guess when you request a flight following they make a note somewhere or mark it in some way.

They tag your code with a destination; for "local" flights, they'll put their own code--and if you leave the area (no longer "local"), they'll drop you.  If you request FF, they'll put your actual destination in, and when you leave the area, they'll try to hand you off.  Also, they have different blocks of codes for different purposes, so if you know the area, you can predict if/where you'll get dropped based on the first digit or two.


text:
Is there a minimum weight requirement for flying manned aircraft in controlled airspace (B, C, D, E)?
The Colomban Cri-Cri is a small twin engine plane with an empty weight of ~200 lbs and a MTOW of ~400 lbs.
Can I fly this in controlled airspace? What are the requirements for aircraft registration?

How do you count lighter-than-air craft? They're allowed into controlled airspace, with a suitable clearance.

It's not a weight thing, it's an equipment thing. Need a transponder, ADS-B-Out (starting Jan 1 2020) and 2 way radio.  OR, calling ahead and making arrangements if not so equipped.
I don't imagine flying something that light around big jets would be very much fun, altho some of them have smaller runways for smaller GA aircraft.
Boston, for example:
https://img.airnav.com/aptdiag/w240/00409.gif?v=MZPOGC

Transponder and ADSB would not be required in Class D or E airspace, and 2 way radio not required in Class E.

Yes,  but the question did not break them out separately.

If there were any minimum weight requirement for manned aircraft in controlled airspace, the requirement would probably be in 14 CFR Part 91, Section 126 through Section 135. These are the sections which describe the operating rules for various types of airspace. I looked through those sections, and there are no weight requirements.
I also can't imagine why there would be a minimum weight requirement for any class of airspace.
Like most aircraft, the Colomban Cri-Cri must have an airworthiness certificate and a registration certificate (14 CFR 91.203). The regulations describing getting an airworthiness certificate are in 14 CFR 21, Subpart H.
Some aircraft weighing less than about 254 pounds are "Part 103 ultralights" and do not need to be registered. However, this doesn't apply to the Colomban Cri-Cri, because Wikipedia says that it has a cruise speed of 100 knots; Part 103 ultralights must be "not capable of more than 55 knots calibrated airspeed at full power in level flight".

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
What does "FG BNK" mean in a metar?
Here's the KLAX METAR 2153Z observation:

KLAX 102153Z 27010KT 10SM FEW002 BKN200 24/16 A2997 RMK AO2 SLP147 FG
  BNK S-NW T02390156 $

I will put my money on "fog bank" without checking...

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What does it mean when there is no ceiling or "CLR" in an airport METAR?
Here's the current METAR at KFLG:

KFLG 210657Z AUTO 21010KT 3/4SM -SN BR VV010 M07/M09 A2961 RMK AO2
  SLP044 P0003 T10721094 410331178

It doesn't have anything about cloud layers or "CLR", but it shows what the vertical visibility is. Why doesn't it have information about the cloud layers?

Vertical Visibility with a number means that cloud layers cannot be determined because of haze, fog or mist, instead it's a measure of how far up the weather station can see. Because its sensors cannot penetrate further the station cannot tell you whether there's clouds above that level or not, but it does say in this case that there are no clouds below 1000 feet.
You may see VV/// sometimes, this means cloud cannot be measured. If fog, mist or haze and poor visibility is reported as well then it's likely that the fog is so thick the sensors cannot penetrate it, if on the other hand visibility is good with no fog or haze it's likely a sensor malfunction.

Since the only difference between fog and clouds is that fog starts from the ground and clouds don't, one can alternatively say that appearance of VV means the ceiling is zero.


text:
Will Tower repeat an initial heading if it's part of my IFR clearance?
If I get an initial heading to fly in my clearance from Clearance Delivery, but the Tower controller does not repeat the heading to fly in my takeoff clearance and I stay on runway heading, can the FAA take enforcement action against me (aka did I do something wrong)?
Is the Tower controller obligated to repeat the heading to fly in my takeoff clearance, or is it expected that I do what was in the clearance issued to me?

Yes, the FAA can and will take action in that scenario. I know because it happened to me :-)
A couple of years ago I was departing IFR from KPDK and my clearance included a right turn after departure. It was a busy and very hot day and I had to sit and hold for release (in a C182 in 100F heat...), so when Tower offered to get me out if I could depart immediately, I jumped at it. Tower cleared me for immediate takeoff without repeating the right turn instruction and I forgot about it in the rush.
I climbed out on runway heading and I started thinking that Tower was waiting a long time to turn me over to Departure. When they did, Departure told me to turn right immediately but I had to say "unable" because I now had obstacles to my right. I was vectored back on course and Departure asked me twice what the Tower had cleared me for, so it was obvious that something had gone wrong although they didn't give me a "possible pilot deviation" warning. Fortunately I was never close to busting the ATL Bravo.
I got home, listened to LiveATC, realized that I'd forgotten the right turn instruction, filed an ASRS report and forgot about it. 89 days later the FAA called to investigate (they have 90 days to act) because ATL Approach had reported it as a "course clearance deviation". Long story short, after a couple of calls and emails they dropped the whole thing without any record because they said it was a trivial incident and I'd shown a positive and "compliant" attitude with them. Although I have to imagine that if I'd busted the ATL Bravo or caused a loss of separation, the outcome might well have been different.
I don't know whether Tower is required to repeat the initial heading instruction (it may be in the ATC Orders), but my experience certainly shows that you can't rely on it. I commented in my ASRS report that if Tower had repeated the instruction it could have helped to prevent the whole issue.

I've never had tower repeat the turn CD gave me; my guess is that if they did that, pilots might start ignoring it and not notice when they gave a different turn.

@StephenS I've never had them repeat it either, and I don't know if it's a good idea or not. In the specific scenario I described I think it would have helped, but that's just me in one particular situation.

A friend of mine was busted when he was given a level off altitude in the initial clearance, 3000ft, then when contacting departure at about 2500 he was told to continue to 7000.  So having received it at 2500 or so they just carried on through 3k, but he was a bit tardy and by the time he'd read back the new altitude, they were PAST 3000, like 33 or 3400 (It was an empty RJ, climbing like a rocket).  The got busted for failing to level off at 3000, because the revised clearance doesn't have "legal" effect until it is read back.  I don't think many pilots know this.

@JohnK Was the pilot busy flying the plane and couldn't read it back immediately? Or was there so much other traffic on freq, and he couldn't get a word in? If the latter, I would understand (as the ATC) if another pilot "cut in" without letting him read it back first.

Not sure of that detail but yeah there could have been another communication going on he had to wait for, and blew past 3000 while waiting.  In any case, as a strict technicality he should have leveled off at 3000, but normally they don't and nobody gets excited.  I had heard a rumor that some controllers at this ATC unit were unhappy with some previous flight crew complaints and were getting hyper sticky with people as a result.  Not totally sure.   Important to know about that technicality though.

It is expected that you will do what was in the clearance issued to me, which you read back as part of the clearance acceptance process, unless the tower modified it prior to giving you clearance to take the runway and depart.

Technically yes. In the absence of any other instruction, the tower would expect you to turn to the heading given in your departure clearance at the appropriate time, which would be after passing 400 ft.  So if you continued on runway heading after 400 ft, the tower controller will likely watch for a while, then if he/she is convinced you are going to continue on runway heading, would probably give an instruction to fly the correct heading and do nothing more, or ask you what you are doing, or something similar.
Whether you get busted for it would depend on a lot of things, like the air traffic unit's policy, your demeanor, whether the controller thought it was an honest minor slip up with no ramifications, or a something that caused a traffic conflict and got people excited, and so on.
That sort of thing was why I used to go out of my way to be super friendly and polite to controllers (a cheerful "good morning" when checking in never hurts as long as it isn't burning up too much air time during busy periods).  Like anybody else, controllers enjoy happy, cheerful professional interactions more than not, and this often buys that little bit of goodwill that makes the difference between getting busted or not in borderline situations.

Turning at 400ft?  In many places that may be a violation of 91.119(b) or (c), if not lethal.  IMHO, turning with inadequate obstacle/surface clearance is not "necessary" when a short delay can make the turn perfectly safe (and legal).  I've never had ATC try to fly me into the side of an office building before, but I'm fairly confident FSDO would side with me for not complying--and if not, at least I'd be alive to complain about the injustice.

Read the CFR more carefully; "except where necessary for takeoff or landing".
Standard IFR obstacle clearance departure requirements, in the absence of any special requirements for a given runway, are no turns below 400 ft, then 200 ft/nm in any direction until above min IFR altitude for the area.  So if I had a heading to fly after departure, and the runway had no special requirements, I'd start turning any time after passing 400 feet.  If there is an office building in the way, it will be reflected in the departure minima for the runway, obviously.

Am I expected to know that for VFR FF?  So far, I've just relied on (what I thought was) common sense for when to make the turn, and nobody's ever complained that I waited too long.

Ah I see the problem... we are talking IFR departures here in the first place so this doesn't really apply.  VFR, you would do what tower tells you or observe the normal VFR procedures in the absence of any instructions, so no worries.

Context was CD giving a turn on departure, which also happens for FF, so I figured their expectations would be the same. I've also flown with some IFR friends, and they seemed to turn about where I would, so I was surprised when you said 400ft.

Remember that for VFR even under positive control, you are responsible for obstacle clearance AND separation from other aircraft, not ATC, and you follow the VFR minimum altitude protocols, not the IFR ones and the instrument departure specs for a runway are irrelevant.  If ATC gave an instruction that you knew visually would be unsafe, it's up to you to tell them you can't comply.

In many cases, Tower CANNOT give you a heading because it will supersede your clearance.
For example, assume you're at KPSP and assigned the Cathedral One departure.

However, right before takeoff, tower says, "fly runway heading." Note that the Cathedral One departure DOES start with runway heading for 31L/R. And yet, it's time to do a victory dance because now you don't have a fly that crazy departure! Tower just gave you a VECTOR!
Ref 7110.65
"Departure Procedures (DP). If an aircraft is vectored off a published Standard Instrument Departure (SID) or Obstacle Departure Procedure (ODP), that vector cancels the DP and ATC becomes responsible for separation from terrain and /or obstructions. IFR aircraft must be assigned an altitude."

If you are given a heading, you must read it back.  If you read it back, you have acknowledged and accepted it, and are required/expected to comply.  If you do not, you are in violation.  If you cannot take the heading assigned, you must say so, and do not accept the heading.
ATC would be all over you if you don't follow the heading that you say you will.  They can report you to the FAA, and they may take action against you.  If there is no immediate risk, likely nothing will happen.  If you cause a near miss, or other real incident, then you'll be more likely to be suspended or fined.
ATC's job is not to repeat instructions to you, it is your job to repeat them to yourself.  Write it down.


text:
What is a land and hold short clearance (LAHSO)?
I was reading this question and I want to know what this term means.

land and hold short clearance

also related http://aviation.stackexchange.com/questions/1967/can-you-go-around-with-lahso-in-effect

It means that you can land on the runway but must hold short (read: do not cross) before some point on the runway.

AIM 4-3-11 Pilot Responsibilities When Conducting Land and Hold Short Operations (LAHSO)
6. A pilot who accepts a LAHSO clearance should land and exit the runway at the first convenient taxiway (unless directed otherwise) before reaching the hold short point. Otherwise, the pilot must stop and hold at the hold short point.

This is generally issued for smaller planes that don't need the full runway and a crossing runway/taxiway is also being used.
As the pilot you have final say in accepting the clearance, if you feel you will be unable to hold short then you should reply "unable" and request full-length landing clearance.

[same link as above]
3. The pilot-in-command has the final authority to accept or decline any land and hold short clearance. The safety and operation of the aircraft remain the responsibility of the pilot. Pilots are expected to decline a LAHSO clearance if they determine it will compromise safety.

Nice answer. I might add two more points for completeness: A pilot with a LAHSO clearance may still go around per AIM 4-3-11(b)(5); and student pilots should not accept a LAHSO clearance per AIM 4-3-11(b)(2). Of course, a pilot should read the AIM section completely before accepting a LAHSO.

For those that may be reading this in the future, the FAA unfortunately killed their HTML versions of the FAR/AIM, so the links in the above comment are dead. The FAA now has a PDF version of the AIM and faraim.org now publishes an HTML version.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Are there ever temporary flight corridors?
A recent question on FAA handling of evacuations prompted me to wonder if there are ever temporary (emergency based) flight corridors set up by any of the governing authorities?
For example, is there ever the airspace equivalent of one way highways set up for a temporary period?

there is nothing stopping ATC from assigning a westbound altitude to a eastbound plane or vice versa (besides common practices)

if you look at the Arrival and Departure routes for a particular airport, you'll see that 1-way corridors already exist to separate departing and arriving aircraft

If the answer is no, there are no reason to have temporary flight corridors set up because the separation rules provide enough airspace as is, I would accept it.

There aren't temporary flight corridors, but during emergencies, the FAA will issue temporary flight restrictions (TFRs), many of which include the following text:

ALL AIRCRAFT ENTERING OR EXITING THE TFR MUST BE ON A DISCRETE CODE ASSIGNED BY AN AIR TRAFFIC...AIRSPACE CONTROL (ATC) FACILITY
AIRCRAFT MUST BE SQUAWKING THE DISCRETE CODE AT ALL TIMES WHILE IN THE TFR.
ALL AIRCRAFT ENTERING OR EXITING THE TFR MUST REMAIN IN TWO-WAY RADIO COMMUNICATIONS WITH ATC.

The controllers will then direct traffic in, out, and within the area as needed.
Also, remember that aircraft separation is by altitude as well as position, and nothing prevents a controller from using all the altitudes in her airspace to vector aircraft in one direction. When assigning an altitude that contradicts the hemispherical rules, ATC will use the words "wrong way" to make sure you understand you are being given an altitude inappropriate for your direction of travel.

Flight corridors (airways) are just standard routes to simplify communication from controllers to pilots when describing where they should fly and between controllers when handing over flights. But the controllers can assign arbitrary vectors (headings and altitudes) to aircraft whenever they see fit.
So there is no need to formally set up a temporary corridor. If the traffic for whatever reason does not fit the usual corridor, the controller will simply assign any free vector and explain the situation to the next controller. They do that all the time anyway.
However in non-radar environment (e.g. oceanic or in case of radar failure) airways become important tool for the air traffic control, because it would not be feasible to keep track of positions of all the aircraft on ad-hoc routes. With airways the controller knows which airways intersect so planes on them need to be separated vertically and only needs to track the aircraft progress in one dimension (along the airway) instead of two (on map).

if by "Flight corridors" you mean federal airways, they were set up (in pre-GPS days) not just to simplify communications but also because they can be flown from NAVAID to NAVAID by the pilot without ATC intervention.

@rbp: You can fly ad-hoc navaid to navaid. But it's even more work to define such route, so airways primarily simplified communication pre-GPS too. And they made ATC work possible without radar as it would not be feasible to keep track of every aircraft position on ad-hoc routes.

note i said "not just to..., but also..."

@rbp: And I am saying it's not "also" but rather a case of.

You could argue that any time an aircraft is given a route which is not on a permanent air corridor, a temporary one is set up, and that if a controller gives several planes a similar route, that corridor is in use
More sensibly, no - there's no need to, there's plenty of airspace in areas with no corridors, and even if there was an unusual amount of traffic the controller would just implement a similar concept (routing aircraft along a similar route in the same direction) on an ad-hoc basis as and when required - there's no need to declare it as a corridor in some official manner and propagate that information to airlines and pilots.
Corridors are only necessary for busy routes, and even then it's not mandatory for them to be used by ATC - it's just sensible to use them most of the time to avoid confusion

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Can helicopters land in a poor visibility scenario?
The Instrument Landing System and some others systems can provide to incoming aircraft guidance to the runway in poor visibility scenarios.
With that in mind, I have these questions:

Can helicopters fly in IMC?
Is there any system that can help the pilot land, just like ILS?

Related

Keep in mind that light helicopters (such as, say, a R44) are not going to be equipped for IFR ops -- IFR helicopters not only must have the full complement of instruments and avionics + redundant power systems, but a stability augmentation system and its attendant hydraulics as well.  (This is even true for the Kaman synchropters with their relative natural stability.  I'm not sure if a coaxial-rotor design would be stable enough to be certified for unaugmented IFR flight or not...)

I clicked on this question because of the title, which I understood slightly differently to the text and existing answers. I'm guessing that a helicopter can land safely in poor visibility (without instruments), but not necessarily in the exact spot where it should land, and provided there are no other aircraft about it collide with. That's true, isn't it?

@UnrecognizedFallingObject  I once flew an R22 kitted out as an instrument trainer which for the reasons you state, was not itself authorised to fly IFR!

@steveverrill Not quite, and it depends on how you understand poor visibility.  A helicopter without instruments cannot fly IFR.  It must therefore be flying VFR and one of the requirements of VFR flight is that you remain in sight of the surface at all times.  If your definition of poor visibility is that you cannot see the landing area, then you cannot fly without instruments.  Thus, unless acting illegally, it is not possible for a helicopter to land, excluding emergencies and pilot error, without knowing precisely where it is. We don't leave not bumping into things to chance.

what is the difference between IFR and IMC?

@GabrielBrito - that's a whole new question, and I believe it's been asked/answered. Do a search before asking, but if you don't find it, go ahead and ask.

@GabrielBrito VFR/IFR are the rules under which you fly. VMC/IMC are the conditions in which you fly. You can fly IFR in VMC or IMC but VFR can only fly in VMC. Visual Flight Rules. Instrument Flight Rules. Visual Meteorological Conditions. Instrument Meteorological Conditions. VMC is flown primarily with reference to the ground. IMC is flown primarily with reference to the instruments

The swiss air force conducted some tests regarding flying in no-visibility conditions (whiteout in snow and brownout in dust) together with the US air force. They tested a solution that projects a digital image of the surrounding environment in the pilot's helmet visor. Here is an interesting video about it: Whiteout Tests - Degraded Visual Environment trials switzerland

Can helicopters fly IFR?  Yes
Do they use ILS?  Yes.  The only tricky part of it is the need to be slow at the end of the glide slope.
An IFR helicopter can use any of the navigation and landing aids available to any other aircraft.
This PPRuNe discussion is interesting.

What's the issue with being slow on the glide slope?

@rbp Because you need to flare at the bottom.  If the approach is busy, you may well be required to maintain high speed until close to the end.  You will need quite an aggressive flare to lose that speed and that may put you above minimums or higher than the glideslope.  Needs careful energy management.  You also need to transition from instruments to visual cues which may be marginal.

@rbp, BTW, perfectly doable, just not straightforward.

Helicopters can fly IFR and they fly the same approaches airplanes do.  At certain airports there will be ILS (or other approach) procedures specific to helicopters.

The difference between the above procedure and the normal ILS 4L is that the copter approach has lower minimums, allowing the helicopters to land in lower visibility than airplanes.  At airports without specific approaches for helicopters, they would fly the same procedures airplanes do.

I am flying AS 365 N3 Helicopter which is a twin engine machine and is approved for IFR operations. The helicopter is fitted with ILS and can fly a normal ILS approach like fixed wing aircrafts of CAT B.
The ILS system fitted on this helicopter takes the helicopter upto 50' above Threshold Touch Down Point and then makes the helicopter fly parallel to runway at 50'.
The approach speed for CAT B aircrafts is 120 kts which can be reduced as the helicopter descends closer to the TTDP. A fwd speed landing of speeds 70 kts and below can safely be executed to avoid a flare in poor visibility conditions. However this is not a standard landing procedure.
In normal conditions, pilot must flare the helicopter to reduce speed and land with zero fwd speed.
In my opinion, a RVR of at least 100 meters is necessary to execute a safe approach with flare at the end for touch down.

Not only do copters fly instrument approaches, they are allowed by regulation to halve both the ceiling and visibility minima for a non-Copter approach (but no less than 1200 and 1/4, or as otherwise restricted).
Also, copters often convert an instrument approach at the Missed Approach Point into a Special VFR clearance, which allows them to fly with 0 visibility (see and avoid) and clear of clouds. For example, this approach into JFK terminated in the middle of NY Harbor at 500 feet, and  shows course lines to the various Manhattan helipads :

How, exactly, does "see and avoid" work in zero visibility?

@Vikki I'm wondering the same thing


text:
Is engine sync to avoid beat frequencies also a concern for jet engines?
Is synchronization of rpm to avoid beat frequencies also a concern on jet engine aircraft like it is on propeller driven airplanes?
Context:
This question on SE-Aviation discusses prop sync for multi engine prop aircraft  to eliminate beat frequencies that can be annoying. But it says nothing about any multi-engined jet aircraft.
On the other hand, this other question on SE-Aviation refers to why it is preferable to run all engines at the same RPM (and hence differential thrust is NOT used for directional control) and I presume the answer applies to both jet and prop aircraft.
Which begs the question above:
Is synchronization of rpm to avoid beat frequencies also a concern on jet engine aircraft like it is on propeller driven airplanes?
If it is, how is this done in typical modern jet aircraft? Is there an automatic system in the FADEC etc?

Some aircraft with turbofans have an engine synchronization system for exactly that reason: reducing the beat resulting from different, but similar, frequencies for the two engines.

MD-80: The McDonnell Douglas MD-80 can synchronize its engines by EPR, N1 or N2 when the autothrottle is engaged:

Engine Synchronizer System
The engine synchronizer system automatically matches the N1 or N2 speed or
  EPR of left engine to match right engine. The system includes a three
  position (N1, OFF, N2) ENG SYNC selector, ENG SYNC ON annunciation, and control
  system with actuator to trim left engine fuel control. The pilot can override
  the system at any time with throttles.
When the ENG SYNC selector is placed in N1 or N2, signals from the tachometer
  generators causes engine synchronizer system to trim left engine fuel control
  until left engine RPM (N1 or N2) matches right engine.

(MD-80 FCOM Sec. 18 Power Plant - General)
CRJ: The Bombardier CRJ-700/-900 can also synchronize by either N1 or N2 via the FADEC:

N1 and N2 Synchronization
In the cruise range, thrust is set to a specific value by the FADEC based on throttle
  position. Synchronization allows the FADEC to match the fan (N1) or the core (N2)
  speed of the two engines for noise reduction. Synchronization is selected by using the
  SYNCH switch on the engine control panel . The left engine is designated as the
  master engine and the right engine will be slaved to it.
For N1 synchronization to be enabled, the right engine N1 must be within 1.5% of the
  left engine N1.
For N2 synchronization to be enabled, the right engine N2 must be within 7.5% of the
  left engine N1.

(CRJ FCOMv1 chapter 20 Power Plant - Thrust Control)

As far as I know, no Boeing or Airbus aircraft has an engine synchronization system. Presumably, the beat is less of a problem for wing mounted engines since the passenger cabin is shielded from direct sound by the wings. For tail mounted engines, engine noise is more of a problem in the cabin.

That really isn't a synchrophase system though.  It's jut the FADEC computers tweaking N1s. There is no adjustment for phase issues.

It's just synchronizing the frequencies, yes. I don't think phase is an issue with turbofans...

@JohnK with a 25 or 30 blade fan, the phase issues are fairly negligible compared with a 4 bladed prop.

I know of at least one large turbofan engine which was a bit close to the margin for fan flutter at some power settings, and the simplest cure was reprogramming the FADEC to keep the fan speed varying continuously within a small range (a small fraction of 1%) to avoid sitting on the exact speed that allowed flutter to slowly build in amplitude over a time period of about a minute. Obviously that would have prevented any exact engine synchronization even if it was desirable for noise reasons (and AFAIK synchronization has no effect on noise, for wing mounted engines).

@alephzero  I would think so also but I'm not sure what motivated GE to develop a fan synchrophaser, at least to the point of patenting it.

Since you mention CRJ, the 200 doesn't have FADEC, and if the pilots let the N1 get out of sync by more than about 0.4% or so, everyone on board will be able to tell.

GE patented a synchrophaser system for turbofans in 1991 but I've never heard of the system being implemented.  My experiences are with the CF34 family (both flying and on the technical side), both FADEC and non FADEC, and none of them had a dedicated synchrophasing system (except for N1 synching by the FADEc computers) and I can't find anything about syncrhophasing on their larger engines like the GE90 either, and PW's geared turbofans also don't appear to use synchrophasing systems (which is a surprise since with the very large gear reduced fans they are a lot closer to being a turboprop).
In any case, the slow woo woo woo of a slightly out of sync pair of turbofans is a lot less objectionable that what you get from propellers, and the crew up front may not even be aware of it, especially if the engines are back at the tail. To the extent that they are aware, or if a passenger complains (more likely), then they would just tweak the N1s a little bit.  It's less of an issue on FADEC engines since the N1 is maintained more precisely and to the extent there is a "woo woo" going on, its oscillation frequency is so low (Woooooo.........wooooooo) that most people aren't bothered by it.


text:
What sort of sensors are used for sensing the positions of the control surfaces for the ECAM F/CTL page on Airbus aircraft?
During the pre-flight flight-controls-check how does the ECAM F/CTL page report on the actual positions of the control surfaces?
What sort of sensors are these? Is it a proximity sensor?
This is motivated by this related question about pre-flight flight-controls-check:
Testing control surfaces pre flight; what feedback does pilot recieve?

No, they are not proximity switches, which are suitable olny to detect when a moving part has reached one of few predetermined positions (as in a landing gear, in which only two predetermined positions – up and down – are of interest; any other intermediate position is transient and undesirable).
For those moving parts that are required to be positioned anywhere within a given range, such as the control surfaces of an aircraft, you need a sensor (aka transducer) that can accurately measure the position of the part at any point within that range, not just at the ends. As already mentioned, the device of choice is the linear variable differential transformer (LVDT), or its rotary variant RVDT.
For example, this is the schematic of a yaw damper servo actuator of an Airbus A320:

Element No. 4 is an LVDT that measures the displacement of the actuating rod attached to the rudder. Similar devices are used for ailerons and elevators. The elevators also have an additional RVDT fitted directly onto the elevator's torque tube, in addition to the LVDT part of the servo control.

Most aircraft I know use either linear transducers LVDTs or rotary transducers RVDTs for this purpose.


text:
What are the advantages of a variable-incidence wing as used in the F-8?
What are the advantages of a variable-incidence wing as used in F-8?
Do weight and other issues outweigh the advantages and that's why it's not found in many (any?) other aircraft?

The main reason for having variable incidence wing in the F-8 was that it allowed the production of increased lift due to a greater angle of attack without reducing pilot visibility as the fuselage stayed level.
For aircraft with high angle of attack TO/landings (like delta winged aircraft), pilot visibility is a problem in low speed landings, as the nose will obstruct the runway (or carrier, for that matter) in extreme cases. There are two ways to solve this problem:

Droop the nose, like the Concorde or
Make the wings variable incidence, like the F-8.

The first option is usually preferred (same thing was done in Tu-144) as the weight penalty is (significantly) less. However, this wasn't possible in the F-8 as the engine air intake was in the nose; so the incidence of wings was changed. The variable incidence wing had the added advantage of reducing the length of landing gear required and improved the low speed characteristics.
In case of Martin XB-51, the variable incidence wings were used to reduce take off distance as it was expected to be operated from forward airfields.

I'm curious as to why do you consider shortening the landing gear an advantage on a naval aircraft. Usually a low-hook attach point creates some problems ( image source ).

@MarcoSanfilippo : just on the case of the F-8 : it was designed with high mounted wings, and therefore required shorter main landing gears (and nose, obviously) Not better, but a design limitation. So why the F-18 has longer landing gears ? F-18 wich is a) twin engined, and b) doesn't have an intake between fuselage and nose tire (cf Navy refusal for a navalized F-16 for the same reasons) But you're right, short landing gears with poor shock absorption is kind of a design flaw for a naval aircraft; as expected, F-8 mishaps are quite numerous.

The Vought F-8 Crusader had a variable incidence wing because of the role it was designed to fulfill.
Both the F-8 and the F-104 had comparable performances, but the F-104 was designed to operate from long runways on land, while the F-8 was a naval fighter.
Higher take-off, approach and landing speed were not an issue on the F-104, so the angle of attack (AoA) was not critical. On landing the '104 could also make use of a drogue chute.
On the other hand on the F-8 the AoA was critical because of the short, cable-arrested landings on the deck of the aircraft carriers.
Considering the relatively small clearance under the fuselage of the F-8, an high AoA during landing could lead to an in-flight engagement of the hook (something definitely undesirable), hence the necessity to keep the fuselage as horizontal as possible.

Tail clearance on take-off and landing and visibility from cockpit.
The aircraft needed rather high angle of attack on take-off and landing. The resulting attitude would:

require long gear,
provide poor visibility to the pilot and
make arrested landings harder, because the centre of gravity would be higher when the hook engages the cable.

So they chose this method of reducing the pitch.

This design feature provided more lift during low speed flight, reducing the approach speed of the F-8 during carrier landings, while providing good aerodynamic characteristics for high speed flight.


text:
How to fold or expand the wingtips on Boeing 777?
I have heard that some Boeing 777's wingtips can be folded when the aircraft is on ground. However, I didn't find the switch to control the wingtips. Where are these switches?
Btw, I use FlightGear and the original 777 from the FlightGear official hangar.

This is only possible in the 777X, not any other 777 variants like the 777-200 or 777-300.
Also questions related to issues in flight simulators are off-topic here.
In the 777X, there is a switch in the overhead panel that can be used to extend and retract the wing tips when required.
It looks like this,

Here's the usage procedure diagram of this feature,

Oh, that's my mistake. Sorry. I'll delete or edit this question later

@GordonZhang, I'm not sure if you need to because your question also has non-flight sim related parts... leave it as is..

All right. I'll keep this question here. Anyway, thank you for answering. At least now I know how to control the wingtips when I have a 777X in Flight sim. :-)

What does "automation FOLDS wingtips" mean, does the switch move automatically as well?

@Bergi yes correct, when the airspeed goes below 50 knots G/S the switch automatically moves into the folded position and the wingtips begin to fold. At set speeds it automatically goes up too
Source: https://theaircurrent.com/technology/how-the-777xs-folding-wing-tips-work/

Is it somewhat ironic that this answer reminds the poster about the on-topic rules for flight simulation, but then includes a screen grab from a flight simulator to support the answer? ;)

@TypeIA, there's plenty of pics of the real thing too, for example the answer below has one...
I felt simulator picture would be fine because the question is asking of a simulator too. If it wasn't related to the simulator at all then I would find and source a real image.

The switch is located on the overhead panel next to the passenger signs:

(image source: LinkedIn: The Boeing 777X Folding WingTip (FWT) operation.)
Note however, that only the Boeing 777X series has the folding wingtips. All previous Boeing 777 models do not have that folding wingtips and therefore no associated switch.
I don't know which variant you are flying in the simulator, but since the 777X is not yet certified, I guess it is an older variant.

It's obviously just a result of unintended humourous cropping, but for a split second my brain wondered what happens when you flip the switch on the right to turn the wings off. ;)

@EndAntisemiticHate Well, that's when you get the once-in-a-career "Wing Off Light" illuminating!

@RalphJ LOL! It's preferable to fly with that light off, but if it goes on... https://www.youtube.com/watch?v=hyCLuDjU57c and https://www.youtube.com/watch?v=wxJcEz3h4tU

What happens to the flight dynamics if you fold the tips while in flight, for example by accidentally flipping the switch when turning off the seatbelt sign? Do they break off or what?

@Richard the switch doesn't work in flight, it's full fly-by-wire meaning a computer controls the wingtips. It won't listen to the switch position unless the weight-on-wheels sensor tells it that the aircraft is on the ground.

@Bianfable - And if it goes off by accident?

@Richard I believe the folding mechanism is also designed so that it can't unlock under load.

@user71659 - Well that's no fun at all


text:
How to find the history of a particular NOTAM?
I have a tower at 1434 feet MSL and it has inoperative tower lights for at least 2 years. I can find a current NOTAM but would like to find out the history of the NOTAM. The current NOTAM is:

NOTAM #: 10/003
Class: Obstruction
Status: Active
Issue Date UTC: 10/06/2024 0856
Start Date UTC: 10/06/2024 0856
End Date UTC: 11/05/2024 0001
!OPN 10/003 OPN OBST TOWER LGT (ASR 1200772) 330411.00N0840839.00W (9.3NM NE OPN) 1434.7FT (315.0FT AGL) U/S 2410060856-2411050001

Thanks for any help on this history

Does that help? Is there a public NOTAM archive that's searchable by airport?


text:
What does this formulation mean?
Just was reading this manual => DLE-120 engine User Manual
On this page (pg 4), I saw this formulation:

What does it mean? How is the 30:1 ratio calculated? What does "fl oz" mean? What does "2-cycle oil" mean? Where are these number coming from?

"fl oz" is a cruel and archaic imperial way of torturing people wanting to measure volume.

An amusing aside: once I watched an early Simpsons episode (the one where Homer's license is suspended due to DUI) with Spanish subtitles on. They translated "128 oz Supperware" (actually volume in fluid ounces) as "3.6 kilograms" rather than "3.78 liters". I think it roughly correlates to 1 fluid ounce of water weighing close to 1 ounce. Ain't Imperial units fun?

Specifically that's a US gallon and a US fluid ounce; there are others, though the Imperial (UK) gallon is no longer widely used; that engine is sold in the UK.  Also note the spurious precision in the metric conversion of the oil (an extra decimal place compared to what the fl oz would give you). As that's a model engine, you're not likely to be mixing vast quantities; one drop of oil is around 0.05ml, so you couldn't be that precise anyway.

What does mean "fl oz"?

"fl oz" means fluid ounce (1 fl oz ≈ 30 ml), which is a unit of volume, as opposed to dry ounce, which is a unit of mass.

How are they calculated the 30:1 ratio?

By ratioing 128:4.26, it is 30.05:1, so 30:1 is a good approximation

What does mean "2-cycle oil"?

It is oil for 2-cycle/stroke piston engines

from where these numbers have came?

Probably some design specs/limitations. You have to ask them if you want the details.

What does it mean?

That when you refuel, for every 30 parts of fuel you put in the tank, you need 1 part of oil for correct lubrication of the engine.

Worth noting: when instructions say 2-cycle or 2-stroke oil, make sure you use this specific oil type. Just any oil will not be ok, in worst cases using wrong type of oil will cause the engine to seize.
I bet some numbnut will rush here to "say I've flown hundreds of hours with just regular 10w40", but, nope, it's not ok:
Wikipedia: Two-stroke oil

Comparing regular lubricating oil with two-stroke oil, the relevant difference is that two-stroke oil must have a much lower ash content. This is required to minimize deposits that tend to form if ash is present in the oil which is burned in the engine's combustion chamber. Additionally a non-2T-specific oil can turn to gum in a matter of days if mixed with gasoline and not immediately consumed.

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

@Abdullah this answer adresses the "What does "2-cycle oil" mean?" portion of the question. Other aspects of the Q are perfectly adressed by Federico's answer.

discussion on meta


text:
Are two little engines or one big engine more efficient for an ultralight?
For an ultralight, should two engines weighing 10kg each and making 7hp each OR one engine weighing 20kg and making 14hp be used?

Does the FAA allow multiple engines in ultralight or LSA categories?

@Harper What makes you think OP, whose profile specifies their location as Nigeria, has any interest in what the FAA has to say about anything?

The Colomban Cri-Cri is a twin-engine homebuilt with 2 15-hp engines. It also has a 4 engine electric model that was built as a demonstrator.

Normally you have more power relative to engine mass in the bigger engine. If there is no weight advantage (like in your example), other considerations must be added:

Two engines allow to spread the load out. By placing one on each wing, the fuselage weight can be reduced which in turn reduces the wing root bending moment.
If the engines drive a propeller, asymmetric thrust and gyroscopic effects can be eliminated with two engines if one engine runs clockwise and the other runs counterclockwise.
Two engines need twice the number of controls, fuel lines and instrumentation. This is both a cost and a weight issue.
And the obvious one: If one of two engines fails, you still have the other to keep you flying. However, this is only true if your design is capable of flying with the remaining engine only. Now your rudder authority and minimum thrust requirement must be adequate to keep the aircraft flying with the resulting asymmetric thrust.

Note also that with two engines an engine failure is statistically twice as likely.

And don't forget 4A. If one of two engines fail, you still have the other to get you to the crash site.

I guess there is a propeller efficiency should be taken into account. If propeller disc diameter is (nearly) same in both cases then one-engine design total efficiency would probably be lower.

Since, Peter Kämpf and the OP presumably considered turboengines (or piston engines), I would like for the sake of future completeness talk about electric engines.
Some concepts of electric powered aircrafts (of small dimensions of course), consider using a fully distributed propoulsion system, involving several engines (see this project from NASA and this project from french ONERA.
So in this case, it seems that in addition to what Peter Kampf said, the more you have engines, the better.

However, decreasing propeller blade span decreases its efficiency, which is currently crucial for electric-powered aircrafts.

@Eugene Indeed, that is probably why the propellers are fitted inside a casing. Also, this is part of the compromise related to the very integration of the engines on the fuselage.

This is all good when you receive power from burning fuel. Although you will have nasty fuel consumption, it will be reasonable considering craft's flexibility. But modern batteries has too little power density compared to fuels, so inefficient designs shorten flight time dramatically.  There is probably a change for on-board generators, but this only contributes to overall inefficiency. Propellers that small (fans, in fact), even ducted, are very inefficient. Although I myself like ducted fans, I admit their inefficiency.

For electric engines, the efficiency chain is for sure more complex to analyse. Between the energy storage, engine efficiency, mass to weight ratios, ... There is much work to improve that ! With so much work on electric drones, I guess we will see some improvements.


text:
What is the average angle of attack of GA airplanes during takeoff?
What is the average angle of attack of GA airplanes during takeoff?

AOA is unknown for most GA airplanes. There is not an AOA indicator in most GA airplanes. The Icon A5 is one of the few airplanes to have one (video here).
5-10 degrees will be shown on the attitude indicator. We go more by speed tho, Vx and Vy. Values can be found in the Pilot Operating Handbook that will be in the plane.
In a Cessna Cardinal for example:
Vy @ 6,000 ft / 2,500 lbs   92 MPH IAS
Vx @ 6,000 ft / 2,500 lbs   69 MPH IAS
This page sums up the characteristics for a Cardinal nicely.
This chapter of an e-book, "See How if Flies" goes into Angle of Attack nicely, trimming the airplane for an AOA, and even has a chart with some typical AOA numbers vs other numbers.
From av8n.com:

The attitude indicator doesn't usually show the angle of attack

Correct, that's why I didn't mention it, and instead deferred to Vx/Vy

The question is about AoA, not attitude indicator. You really should explicit the relatioship between AoA, Vx/Vy and attitude.

Depends on how slow you are going at rotation, and how aggressively you rotate.
Stalling AOA of most GA wings is somewhere in the region of 14-16 degrees and on takeoff it shouldn't get above 10ish to have adequate margin.
The issues over taking off with frost is due to the frost reducing stalling AOA to an unnaturally low angle, say 9 degrees, so that the pilot gets a surprise during a normal pitch up.

If the t/o aoa is 10°+,how about descent?What is often the aoa during descent? @John K

Your question title says AOA during Climb, but your question says during takeoff.
About all that can be said about AOA during takeoff is that it starts out very low, (in tricycle aircraft anyway, in tail dragger it starts out high until the pilot pops the tail off the ground) then, at rotation, increases to something higher than climb AOA, but lower than stall AOA, then decreases as the aircraft accelerates towards climb speed.
In normal climb procedures published for all aircraft, including General Aviation (GA) aircraft, it is recommended that climbs be flown at the airspeed that produces or generates the maximum possible rate of climb, this is normally referred to as $V_X$. and it is calculated based on gross weight to put the aircraft at the angle of attack (AOA) that produces the maximum excess power. This occurs at the point where the ratio of Lift to Drag is at a maximum $(L/D)_{Max}$.
 SO, the answer is the AOA Is that AOA that produces maximum excess power or $(L/D)_{Max}$.


text:
What creates most drag during flight: fuselage or wings?

From pic 1, I know the drag line is below the thrust line, but I am puzzled about where the drag line is in pic 2?
Secondly, what creates most of the drag in flight, is it the fuselage or the wings?

Unfortunate registration of the first plane.

It's the wings. Most of the time.
Best L/D is reached when the induced drag is equal to the non-lift-related drag. Induced drag is a byproduct of lift creation, and that is predominantly done by the wings. Therefore, induced drag (physically a backward tilt of the lift vector away from the direction in which lift is defined) is produced by the wing.
For the remaining drag you can do a very rough estimation based on wetted surface and friction drag. Here the wing again produces a substantial fraction which must be added to its induced drag.
For racing aircraft like the Supermarine S.6A (which won the Schneider Cup back in 1931), the fuselage, struts and the floats did produce almost all drag at high speed. This would be a clear exception to the answer given above. However, even with this aircraft, at low speed (during take-off and landing) the wing would most likely create at least half of the drag force.
On the Dornier Seastar (your first example) the thrust line is chosen so high in order to move the propellers and turbine intakes away from possible splashes of water. The resulting pitch moment is trimmed away with proper elevator deflection. In your second example a lot of drag is created by the many bracing wires. At low speed the wing will produce most of the drag while at high speed the drag line will move down when the unfaired pilot (and wheels, engine and tank) and the many wires will create most of the drag.

Hi Peter, from what I've seen so far, seems like about 30-40% of total drag is the induced drag for an average jet transport in cruise. Is this range (30-40%) also true for slower flying turboprops? Interestingly, according to pilot manual, even old birds designed in the 50-s, like An-24, had aspect ratios of more than 11.

@ElectricPilot Google "Davis Wing" - the B-24 and B-29 also had aspect ratios above 10 and one Messerschmitt design even above 14. For ideal efficiency induced drag should be half of total drag for turboprops.


text:
Why are the first sounds of an approaching enroute airliner low frequency, booming ones?
Often I will hear an intermittent, low frequency, powerful, booming sound that resolves into that of an airliner flying enroute high (FL 300+) overhead. So the first sounds that an airliner flying overhead to be heard are those. Why? I assume it has to do with the atmospheric conditions and how sound waves of different frequencies propagate forward..

Related answer to Why is a Lockheed MC-130J Commando II creating such a loud droning sound?

At some locations like Point Of The Mountain near Salt Lake City an airliner is typically heard as a very abruptly-starting loud whine rapidly descending in pitch. Is your location mountainous or flat?

Related: What makes the swooping noise when a jet flies overhead?

@quiet flyer I first noticed it in northern NH, then sometimes in southern NH also.

The air attenuates high frequencies more than it does low. Thus, lower frequency sounds travel farther, so you hear them first. Also, longer wavelengths will appear to "bend" around obstacles more than ones with shorter wavelengths, so buildings around you will in many cases develop acoustic "shadow" zones for higher frequencies as those buildings' dimensions are significantly larger than their wavelengths.

I think there's also an effect where sound is deflected upwards due to temperature gradients, and high frequencies are affected more than low.

Low frequencies are attenuated less over distance and by obstacles. The lower frequencies, even from the music from a dance hall or band, or that car near you playing loud music will be heard first. It may be all you hear!
It is also common that as we get older, and especially those in aviation, we become high tone deaf!!

If you live in the approach path to a big airport, then there will be thumps and muffled booms as the landing gear is extended. It could be this that you are hearing.

Yes, I've heard those. I edited my OP to clarify that I'm referring to an airliner cruising up high.


text:
Is transponder required to turn off in formation flying?
When planes are flying in formation, TCAS will undoubtedly alert the crew. As a result, in this instance, the transponder must be turned off. The pilot must, however, rely on eyesight rather than displays and sensors which can be difficult for them to fly closely. Please correct me if I am wrong until this point? Is it therefore necessary to turn off the transponder when flying in formation? Or pilots are instructed to fly their plane by a certain speed and attitude at a particular location beforehand?

See also: In a part 91 formation flight within a mode C veil, are all the aircraft required to have their transponders on?
Yes, the transponder should be off/standby.  From AIM 4-1-20-a-4-c

(c) When participating in a VFR formation flight that is not receiving
ATC services, only the lead aircraft should operate their transponder
and ADS-B Out. All other aircraft should disable transponder and ADS-B
transmissions once established within the formation.
NOTE-
If the formation flight is receiving ATC services, pilots can expect
ATC to direct all non-lead aircraft to STOP SQUAWK, and should not do
so until instructed.

Correct in most low-altitude areas, but I don't believe the AIM can be used as a justification to violate the FARs. Namely: within 30NM of a Class B airport, above Class B or Class C airspace, and above 10,000 MSL, all aircraft should have an operating transponder set to report Mode C altitude unless specifically instructed otherwise by ATC. (On our side we won't get alarms for two 1200 codes right on top of each other, while we do get alarms for a 1200 code conflicting with a discrete code.)

When I wrote this, I forgot to check for existing answers.  https://aviation.stackexchange.com/questions/56490/in-a-part-91-formation-flight-within-a-mode-c-veil-are-all-the-aircraft-require is slightly different in scope, but has information on the ATC side.

@BowlOfRed So, Do pilots feel relatively any difference (inconvenience) after turning off their transponder?

during this flight?


text:
What is that to and fro moving thing?
This video by AgentJayZ is the maintenance and repair of AI-25 turbofan. What is that thing moving due to applying certain pressure to that pipe? Can anyone describe its purpose and how it works?

Reminder: please no answers in comments.

Related: this and this. I can't say more.

@mins I am not sure I get an answer on how do bleed valve actually works? How can it sense now it is time to suck before the occurrence of the stall?

@Auberron Comments are not for answers, not even partial ones or assumptions for answers. See the help center. If you want to have an informal chat about your question, use a chat room.

once again; comments are for seeking clarification and improving the question. Do not post pseudo answers in comments. they will be deleted. If you wish to answer the question, do so in the answer box below.

@Jamiec: You're right, it's in the book, but yourself you also like to comment, provide anecdotes and even jokes, and sometimes clues for the answer. And frankly I don't see a problem, these elements help others to provide more complete answers. If some cleaning is needed it's always possible to do it later.

@mins If im guilty of it, I would expect those to be deleted also. I am human, I do make mistakes.


text:
Why generation of lift is more at mid span of the wing?

For the same condition like same density, wing area, free-stream velocity, From the above figure one can see more pressure differential and hence more lift at mid span of the wing?
What causes this phenomenon? Is it due to influence of tip vortex?

The diagram might be a bit more clear if a fuselage were attached to one of the sides.
If that were the case, lift would be less on one side because of the fuselage is interfering with the air flow.  Lift would be less at the other end due to washout (lowering of wing tip angle of incidence), and effects of higher pressure spilling from the bottom around the wing tip towards the lower pressure on the upper wing (vortex formation).
The airstream in the middle of the wing experiences neither of these two interferences, generating lift most efficiently.
This is why gliders favor very long (high aspect) wings, where most of the lift is like the "middle" of the diagram.


text:
What is the rod attached to the top of the Bell UH-1N's rotors?
So, I was looking at an image of a Bell UH-1N, and noticed that the top if the rotor has a little stick or rod
What is this rod for? I also see this rod on a RC helicopter.

It is a stabilizer bar used on some two blade helicopters. The spinning weights want to stay in one plane and are used in conjunction with the cyclic to control changes in pitch and roll. Depending on how it is designed, it counters these changes making the helicopter more stable (less maneuverable).

Means its less input needed on the pilots side?

@lpydawa Means slower response, so less twitchy.

is it to reduce the chance of pilot induced oscillation?

@lpydawa PIO results from overcontrolling, so a slower response from the aircraft probably makes PIO worse.

why would they want a slower response then?

The bar is a gyro which wants to remain level.  Its action on the rotor blade pitch angle mechanism causes the rotor itself to tend to seek a level plane of rotation, absent any input from the pilot's linkage, giving the helicopter's rotor a bit of stability it wouldn't normally have.  You could call it a "self-leveling servo" device.  So as Pilothead says, less twitchy, and a bit less tendency to wander off in any direction at any time.  In the Huey Cobra, which is based on the same machine, the bar was done away with because responsiveness was a priority.

@lpydawa What John said. It is a design choice, like how trucks handle differently than race cars. If you let go of the steering wheel of a truck it will go straight until it runs out of gas. If you let go of a formula1 wheel, the first crack in the pavement will send it into the weeds.

@lpydawa To avoid over-controlling the helicopter.  To provide more generic in flight stability.  See this answer about stability versus maneuverability.

@JohnK that looks like a separate answer


text:
How does the Boeing 777's yoke of both the captain and the first officer have synchronized movement?
On Airbus planes, the side stick of both the captain and co-pilot are not synchronized in movement, meaning if you the captain moves the side stick, the co-pilot's side stick will not move together. If the co-pilot moves the side stick while the captain does, it will be a dual input.
However, on the Boeing 777 is also a fly-by-wire(FBW) design. But if you watch a 777 cockpit during flight, both of the captain and the co-pilot's yoke moves.
How is that done? Are both of the yokes connected by one mechanical system, and that mechanical system feeds the pilot input to the conputer? Or is it done electronically, with motors or other stuff?

From the FCOM v2 (9.20.1):

The primary flight control system uses conventional control wheel, column, and
  pedal inputs from the pilot to electronically command the flight control surfaces.
  The system provides conventional control feel and pitch responses to speed and
  trim changes. The system electronic components provide enhanced handling
  qualities and reduce pilot workload.
[...]
The columns and wheels are connected through jam override mechanisms. If a
  jam occurs in a column or wheel, the pilots can maintain control by applying force
  to the other column or wheel to overcome the jam.
The rudder pedals are rigidly connected between the two sides.

So yes, there is a direct mechanical link between the control columns, but it can be overridden.

Edit:
As John K pointed out in the comments, the technical term for the mechanical link is a torque tube with a breakout mechanism. The details are described in patent US5782436A:

When a sufficient amount of force is exerted on the un-jammed control, the connection of the swivel arm to one side of the torque tube will disconnect due to the force applied overcoming the force of the spring holding connection in place.

What does it mean by "jam"

So basically the movement on the yoke is electronically created?

No, they are linked mechanically, but the link can broken with adequate force thus allowing the free control wheel or column to control the aircraft.

ohhh I see, do they designed it so it is linked, but if you apply force then the mechanical system (lets say its a locking / electromagnetic system) will automatically toggle off.

@lpydawa To "jam" means to get stuck (this is a non-technical term).

"both of the yokes connected by one mechanical system, and that mechanical system feeds the pilot input to the computer" - just like non-FBW airplanes.

So are you saying the 777 is not fbw?

@lpydawa no its still FBW, but there is a mechanical linkage between the two yokes to mirror movements, beyond the cockpit it is electronic .

@JamesTrotter I see, mechanical as in like a locking system or electromagnetic?

The FBW system is more or less replacing the control cable runs and most of the bellcranks and quadrants with electronics.  Being keeps the traditional controls and linkages in place "upstream" of where the cable runs would start, all to run the little electric transducers that tell the computers were the controls are, because Boeing's philosophy for FBW is to physically replicate the traditional config as much as possible, and give the crew more authority than the Airbus system does.

The columns will be interconnected by a torque tube that is split into left and right ends, with a breakout clutch device making them operate as a single tube until a certain amount of force is applied.  With all the mechanical linkage in the cockpit, provision still has to be made for things like seized bearings and such, hence the anti-jam provisions that are similar to a non-FBW system.

@JohnK Please post that as an answer instead.

We're looking for answers that provide some detail. Just picking one of the two alternatives suggested by the question, and giving no justification for it, isn't an answer.


text:
Is a propeller with two long blades more efficient than one with three shorter blades?

Is a longer, two-bladed propeller more efficient than a three-bladed propeller with a smaller diameter?

Can you please edit your question to clearly state what you are trying to ask?

Already answered here: https://aviation.stackexchange.com/questions/23009/what-are-the-advantages-of-more-than-4-propeller-blades

As Troy E and Dave said, most of this question is already answered here.
In addition to this previous answer, just remember that a propeller is a technological solution to a simple question: how do we make the aircraft fly ? Therefore, there is no single solution, but multiple compromises that can be found depending on various parameters, mostly based on the type of operations the aircraft should take part in :

Speed
Altitude
Mass of the aircraft
FOD / injestion risks and tolerance
...

Since your question only states the efficiency of the propeller, you actually miss at least part of the real problem.
Let's say you manage to make a propeller with an efficiency of 1, but that it weighs 1 ton or is impossible to manufacture, you won't get far.
In that sense, only considering the efficiency to say what is the best solution is not good enough. The best solution is the best compromise between all of your requirements, that's why it is so difficult to find ;) !

I don't have a definitive source, but I read this recently:
"The rule of thumb is that you need 100 HP to justify each blade. Two blades are considered adequate through 200 HP. Three-bladed props seem to come into their own on efficiency at or above 300 HP. "


text:
If operating an ultralight with private pilot certificate, which rules apply?
If a holder of a private, commercial, or ATP pilot certificate operates an ultralight aircraft; and that aircraft is also equipped with the radios and transponders required by an airspace for GA aircraft, can this flight operate under standard VFR rules as a GA aircraft rather than operating under ultralight rules? (ie. Using non-towered airports which are marked "ultralights prohibited" in the chart supplement, and controlled airspace airports.)
All of the regulation of airspace that I can find is focused on either the pilot's certificate and rating, or the aircraft equipment, rather than the aircraft category and class. That is a slow "experimental" light-sport STOL bush plane (which could be as slow as an ultralight), a C172, or a King Air 250 are all equally allowed into class E, D, C, or B airports as long as they have the required electronics and a PIC with a private pilot certificate or above.
I ask mainly because I would like to use an ultralight for commuting, and the places I commute are airports of the above nature. (local street traffic is terrible, and I need STOL capability for my home field. Less than 700ft to clear a 50ft obstacle.)

Per 14 CFR Part 103, an ultralight can not be registered as an aircraft under Part 91. But, there is no regulation limiting the weight of an aircraft being operated under Part 91. To do so, an aircraft just must be registered under Part 91 as an aircraft. But, an aircraft can only meet either Part 103 or Part 91. It can not meet both simultaneously. In other words, an ultralight that meets Part 91 regulations is no longer an ultralight. Although, it could be considered a Light Sport Aircraft or experimental aircraft.
If an aircraft were to operate under Part 91, it must meet all aspects of Part 91 regardless of its weight. This includes all documents, inspections, maintenance, and equipment. You will not be able to cherry-pick the ones that suit you. But, for flight time used to meet Part 61 requirements at this time, no time flown under Part 103 can be used.
The better question is whether you can operate your ultralight in the airspace that you want to. Per 14 CFR Part 103.17, you can operate an ultralight in any A, B, C, D and E to the surface airspace for which you have prior ATC authorization. There is no time period or mechanism specified for the authorization. But, if you use a radio onboard your ultralight to ask for authorization from ATC, will it have the wattage to maintain two-way communication?

If the aircraft is not registered, it can only be operated under part 103.
FAR 103.1 states that an ultralight may not "have any U.S. or foreign airworthiness certificate".  So a registered aircraft could not be operated under Part 103.
Your question mentioned transponders.  Bear in mind that FAR 91.1(e) exempts ultralight vehicles from all of Part 91.  Therefore, if operating under Part 103, there is no particular airspace that would become available if a transponder were added to the ultralight, that would not be available otherwise, unless a particular ATC facility chose to grant the "prior authorization" required for Part 103 operations in certain controlled airspace, specifically on that basis.  This seems unlikely.  In fact, it is probably safe to assume that the FAA would attempt to prohibit a transponder from being added to an aircraft operating under Part 103, although it's not clear what regulation this would be based on.
Related:
In the United States, are ultralights legally considered aircraft?
Would FAR 91.227, or any other regulation, forbid use of ADS-B-out in an unregistered ultralight?


text:
Is the shear centre of a wing the same as its aerodynamic centre?
How can the shear centre of a wing be located and is it the same as the aerodynamic centre of a wing?
Lastly, I know how the centre of pressure interacts with the aerodynamic centre but not the wing's shear centre, so how does the centre of pressure relate to the shear centre of a wing during flight?

No. These are very different things.
The shear centre (SC) is a purely mechanical (structural) feature of the wing design. It is a point along the chord such that application of force - no matter what force - causes the wing only to bend but not twist.
Unlike the aerodynamic centre (AC) and centre of pressure (CP), it makes no sense to talk about a single integral point for the whole aircraft. An SC can be found for each cross-section of the wing, and it is the chordwise relation of the SC to CP and CM (centre of mass) of the section that is important to aircraft designers.
Designers can control where the SC will be. If we could make the SC match the point of application of the integral force at this section, we wouldn't need much of the torsional rigidity, and could make the wing lighter.
Unfortunately, this is not practically possible. First and foremost, the CP, the integral point of application of the aerodynamic force in flight, moves forward and backward depending on the angle of attack (AoA). This is just the nature of aerodynamics.
Typically, for a straight wing CP will be behind SC for small angles of attack and ahead of it for large angles. This is rather unfortunate, because this means that at a high G load (esp. at moderate speeds), or in an updraft, the wing will twist to an even higher AoA. This positive twist at the max load (typically 1.5 × rated load) is often one of the critical design cases, so in practice we want SC closer to the forward-most CP location. A clever application of SC can make the wing twist negatively and dampen its own oscillations.
CM of the wing section is nearly always behind the SC. In normal flight, static aerodynamic loads prevail over weight of the wing itself, so this doesn't matter much. However, when we start analysing aeroelastic modes, CM location becomes important. To avoid flutter, a common solution is to move CM closer to SC by installing some dead mass at the leading edge. This often proves to be easier and lighter than increasing torsional stiffness. If we can, we would install some useful mass there, such as an engine.
I didn't talk about AC yet. This is because it has nothing to do here. AC is a very theoretical point introduced for convenience of stability analysis. Any force can be broken into a moment and another force, and we just split lift such that part of it acts at CM, and part at another point. Because CP normally moves linearly with AoA, we can make it such that all changes to lift due to AoA 'kind of' happen at a fixed location. But in reality, of course, lift just increases and moves forward with AoA. The structure and aerodynamics have no idea about our mental exercise.
It is perhaps this fixed location of AC that lets it be confused with SC, which is also fixed. But they have nothing to do with each other. Structural engineers don't need to know about AC at all (they just need to ensure the aircraft CM is in a certain range they are told), whereas flight dynamicists hardly ever talk about CP but always use AC as the reference point (along with CM).


text:
Is it OK to use foam for the ribs of an ultralight's wing?

I have seen several YouTube videos of ultralights and even HPAs (Human Powered Aircraft) that use foam as the ribs  of their wings. Are ribs not supposed to be made of rigid, strong, woody and light material i.e pine,balsa etc.?

Isn't wood basically a foam at the microscopic level?  It's composed of once-living but now empty cells, with cell walls of cellulose & lignin.

@jamesqf; No, it isn't. Not at all. It has high anisotropy imparted by oriented cellulose fibers.

The answer depends on the wing loading of the aircraft. Human powered aircraft need to have a low wing loading so very little energy is needed to sustain flight. Consequently, the loads on the single structural members are low, too, so you can get away with foam-only ribs.
Next, the answer depends on the function of the rib. If it sits at some mid-span point and its only function is to keep the wing skin in the right shape, loads on the rib are indeed low. However, ribs at the wing root of a removable wing sometimes have to carry half the lift load, for example in gliders. There a foam rib would be totally inadequate.
You need to determine the local loads and then decide if foam alone will be sufficient. If not, consider strengthening the foam rib with reinforcements. Essentially, you use the foam as a scaffold and place stiffer material where the sticks on the wooden rib go.

The function of the diagonal members (to carry shear loads) can be performed by ±45° glass fabric on the surface of the rib.
Lengthwise reinforcements using glass rovings on the top and bottom side of the rib improve its bending strength.

The foam is used to take small shearloads and support the primary structural element in absorbing tension and compression loads.  The wood rib has the primary compression and tension members in the capstrip, with the diagonals absorbing shear loads, converted to compression and tension loads along the legs of the truss. On a foam rib, if it is a standalone rib not connecting to a stressed skin, you need to have a structural capstrip material, like wood or fibreglass because the foam only has strength when the loads are distributed over a large area.  The foam itself does the job of the diagonal members in holding the caps in place and absorbing shear forces in the rib.  Some ultralights have a foam rib core with an aluminum cap strip bonded to the edges to do this.
If the foam rib is inside a stressed skin wing, the stressed skin is effectively the cap strip and the foam can be bonded directly to it.  Because the foam isn't very strong it requires a large number of foam ribs, closely spaced to properly support the stressed skin.


text:
What makes the Antonov An-2 have no stall speed?
The Antonov An-2 is notable for being able to fly at a very low speed. However, what confuses me is that it has no stall speed:

The An-2 has no stall speed, a fact which is quoted in the operating handbook. A note from the pilot's handbook reads: "If the engine quits in instrument conditions or at night, the pilot should pull the control column full aft and keep the wings level. The leading-edge slats will snap out at about 64 km/h (40 mph) and when the airplane slows to a forward speed of about 40 km/h (25 mph), the airplane will sink at about a parachute descent rate until the aircraft hits the ground."

(From Wikipedia article on An-2)
How is this possible? What makes the airplane have no stall speed?

Well, that's not the same as saying there is no stall speed.  Rather, the manual seems to be saying that the stall speed is very low, and pulling the stick or yoke full aft does not provide sufficient control authority to reach the stall speed.

OK, I guess more recent answer suggests stick full aft does produce sort of a stall, but a nose-high mushing stall not a severe nose-drop stall.

The horizontal tail can't make sufficient down force to slow the plane to get the slatted wings to stalling AOA.  With the slat system, the stalling AOA will be perhaps 10 degrees higher than without.  With the AN2's large wing area, to get the slatted wings to stalling AOA might require the speed to drop to, say, 20 kt.  But the tail simply runs out of down-push before that angle is reached, in this case at 25 kt, so no stall can occur and the airplane settles into a near vertical mush as long as you hold the full up elevator.
Because there is no actual stall break or departure, the published "stall speed" is the lowest speed that airplane can be flown and maintain altitude.
Note that a "parachute descent rate" is about 20-25 fps, or 1200-1500 fpm.  That's still coming down pretty fast (you would probably collapse the gear landing normally at that descent rate) and the airplane will be wrecked when it hits the ground, but you'll live.
The interesting thing is that if you do get a slatted wing to actually stall, the departure can be very violent.  An acquaintance who flew a Pegazair he built, which has automatic slats, was horsing it around and was able to get an aerodynamic stall by using large elevator pulsations as he was approaching minimum speed, reported that it flipped over violently and his flight bag flew from the baggage well behind him up to around the windshield.  It recovered fine however.

This site states it a bit better:

Stall Speed: 35-40kts. but controlled descents are possible at 25kts. or 30mph.

In reality most aircraft have the ability to glide at some speed. The AN-2 simply has the combination of a quite a good glide ratio and a low stall speed so the aircraft can be settled to the ground in a controlled manner.

How is a controlled descent possible at below stall speed?

All airplanes still produce lift even when stalled; the An-2 produces enough of it to remain in controlled flight even in a stall.

@vidarlo There are various definitions of stall. I'm guessing the definition used here is one where the wings no longer generate sufficient lift to maintain altitude - which is technically stalling the wing. However this does not mean lost of aerodynamic control - which is the other common definition of "stall". Planes can be designed with oversized and/or slotted control surfaces or with delta wings or have vectored thrust to be able to do post-stall maneuvers

@slebetman A stall is defined as the reduction of lift as a consequence of exceeding the critical angle of attack. The loss-of-control 'definition' conflates and confuses a possible and important consequence of a stall for the stall itself... I say 'possible' because if loss of aerodynamic control was inevitable, it would be impossible to recover from any stall.


text:
Can MIL-PRF-5606H hydraulic fluid be used in place of MIL-PRF-5606?
Can MIL-PRF-5606H hydraulic fluid be used in place of MIL-PRF-5606?  My airplane is a 1966 Piper Cherokee 6.

Hello David Allen, welcome to aviation.stackexchange!

Kind of related: What is the difference between MIL-H-5606 and MIL-PRF-5606 hydraulic fluid?


text:
Why do some FAA letters of interpretation appear to be removed from the LOI database without any apparent explanation?
I've encountered scenarios over the years in which a person has an old legal interpretation saved somewhere on their computer or a private server on the internet that they whip out during an internet debate about some nuance of the regulations, but when the FAA LOI Database is searched for the letter, it is absent.
Here is a great example:
The August 22, 2006 interpretation in response to K. Kortokrax.
I found that link via  a Google search, and it is indeed even hosted on the faa.gov web domain.
However, when I search the FAA's "Interpretations Search" page, it appears to skip over this letter altogether with no explanation as to why.
I have not heard any rumor that the provisions of this interpretation no longer hold, such as the notion that an instructor and student are not considered to be each other's passenger.
Why do letters of interpretation fall away in this manner?  How can we know the reason? I assume that a letter which is replaced by a newer interpretation would be taken down, but by what process to keep air-missioners up to speed?
One example I know of is the debate sparked by the 2008 Glaser interpretation in which the question was posed -- "May a radar-based approach such a ASR or PAR be used to satisfy one of the 'three kinds of approaches' required on an instrument-rating prerequisite cross-country flight?" -- The response, in an attempt to emphasize the onboard-equipment nature of the navigation equipment to be used, stated that "three kinds of navigation systems" were required.  Some in the community understood this as a misspeaking of the counsel in the context in which the response was intended to be used (as a response to the question about radar approaches only), while others took this new information into unrelated contexts and began touting that one could not use a localizer approach and an ILS on the required flight because they use the same navigation system.
The FAA eventually corrected this debate by issuing a memorandum that directly cancels that interpretation:

That above example is rather pronounced.  What are some other ways the FAA changes stances on LOIs while keeping the public informed?  Do they do "silent redactions" by simply taking the letter down from public view without publicly stating their change in opinion?  Would that explain the disappearance of the 2006 Kortokrax interpretation along with many others?

Voting to close because the question is asking for opinions.

@JuanJimenez nothing here is soliciting opinions.  A person either knows the possible mechanisms of LOI posting and removal, or they don't.  If someone doesn't know, yet they choose to provide a speculative answer, then that answer should be handled appropriately.  By asserting that this question is requesting opinions, you're implying a belief that it cannot be factually-answered, which isn't the case because it most certainly can be by a person with the knowledge of the inner workings of the office of the chief counsel at the FAA.

The only way to get an authoritative answer to this question is to ask the FAA's legal department, and they likely will not give it to you. Everything else is opinion or speculation. My statement stands.

The Kortokrax (2006) letter can still be found on DRS, but its status is listed as "Historical." It's unclear why- I can't find another interpretation rescinding it, and indeed I can find several referencing it. Some letters that have been specifically rescinded also show up in DRS as Historical.

@RyanMortensen Yeah. I guess a lot of flight instructors are gonna have to get night current now!

This doesn't cover the full scope of all possible ways the FAA can inform the public of a rescinded LOI, but it turns out that the day before I posted this question, a formal Memorandum from the FAA was released that informed the public that the 2006 Kortokrax legal interpretation that I used as an example of a "missing" LOI, was indeed rescinded back on July 23, 2023, but the memo informing the public is dated August 16, 2024, nearly 13 months later.
https://www.faa.gov/media/84641


text:
Why is a combustor in a gas turbine engine asymmetric?
As the above question stated, the upper and lower section of the combustor isn't same. Is there any reason behind it? Due to which the cooling of the combustor is also different in the upper and lower section of the combustor.
The below picture clearly depicts it:

In order to keep the outer diameter constant, the diffusor expands the airflow more towards the center since there is plenty of space available.


text:
At which angle tiltrotor's rotors generates more lift?

                         [Tiltrotor 1]

                        [Tiltrotor 2]
I can't name these tiltrotors. I just want to know which of the tiltrotors generate more lift considering aerodynamics and based upon the rotors horizontally tilted or vertically. I used these images just to show an orientation of rotary wings at which angle they are tilted not for differentiating these two models. And what happens to the lift generated by the rotors tilted 45 degree to the axes.
Note: Only concerned with the rotary wing either horizontal or vertical inclined but not whole aircraft design.

Technically, if the tilt rotor flies forwards with vertical rotors, the lift is produced by the wings and forward thrust by the rotors. So let's regard the rotor thrust, whether in horizontal or vertical rotor position. Tilt of the rotor does not affect the amount of thrust produced. The following factors do:

Rotor speed. This would remain pretty constant over the tilt range, with tip speed as close as possible to drag divergence speed. At top speed with vertical rotors, rotor speed must be lower than in the hover because of the velocity triangle tip speed = rotor speed + air speed.
Blade Angle of Attack. A function of blade angle and of airstream inflow.
Airstream inflow. With the rotors in a vertical position and flying forwards, air flows into the rotor disk and requires less acceleration to provide the same amount of thrust. Or: a given thrust requires less engine power in forward flight than in vertical take-off.

Therefore, for a given amount of engine power the vertical rotor at speed would be able to provide the highest thrust - but depending on the max. speed of the craft, rotor speed must be reduced, which also reduces the ability of the rotor to convert power into thrust.

The picture above from Leishman, also used in this answer, shows the amount of engine power required vs. airspeed for helicopters. Note that a helicopter rotor must always provide more thrust than its weight. In order to gain airspeed, the main rotor is tilted as well, and the effect or airflow at higher speed can be seen from the reduced amount of induced power.
For a tiltrotor, the rotors will be dimensioned for vertical take-off: they must provide thrust > weight in this situation. In horizontal flight the wings provide the vertical force for weight compensation, and the rotors only need to overcome drag. Which increases quadratically with speed, so at top speed thrust will be the same as TO weight again.


text:
What are those tiny circles on fan blades of Trent 1000?
From Sam Chui's video, there are tiny circles on the surface of three fan blades of next generation Trent 1000 engine. Why aren't they present on remaining fan blades?
Below comments suggest they are for optical purposes. What optical information can be inferred from these circle?
My prediction: They are embedded explosive charger to test explosion testing(specifically Blade off testing of an engine.

At about 14:11 in the linked video the engineer talks about installing an explosive bolt to test engine containment. He's not referring to the small circles, so whatever they are for, it's not that!

Explosive charges would be at the root of the blade, not distributed of the entire Surface. They could be static pressure measurement points or strain gauges. But I suspect they are markers for an optical measurement system.

my guess is that they're nails to keep the blades in place with the frame, but I'm not sure.

@DeltaLima since they are on both the composite painted surfaces and the stainless leading edges I'm going with stickers for some optical purpose also.

I'm guessing that they are all on a slightly different radius.  That would make it easier for a laser to focus on one at a time and so measure defomation/vibration under load at that point.

These are certainly targets for an optical CMM (coordinate measuring machine) using laser, structured light or stereoscopy. The targets help define common points on the model from one scan to the next.
@mins linked a perfect example of this in his comment below, from which this image is taken. When doing 3D scanning, you want the dots in a random pattern as in the OPs image, rather than the uniform pattern seen below. This helps the identify and match the dots when going from one scan to the next (called "tie-points"). For the vibration analysis process linked by @mins, the requirements are probably different as the camera doesn't move, so it makes sense to place the dots in a grid pattern.

What is a "100% target"?

@eleventyone: I think the poster means "It is 100% certain that these are targets..."

So do I, but what I think is irrelevant.

Likely used with ARAMIS sensors and software for deformation/vibration analysis.

Before anyone comments that they seem to be randomly placed: the "randomness" makes it possible to identify each blade from a scan. The blades are also numbered, but this pattern variation is another blade indentification method.

@Jpe61 absolutely! I tried to cover that in my answer.

You updated you aswer just as i posted my comment


text:
Why is ram air not used for air conditioning in airplanes?
If the ram air is used for air conditioning, the airplane doesn't need a heat exchanger, cooling fan and cool the bleed air from compressor. So why do modern planes use bleed air cooled by ram air?
My guess would be mass of the air. The compressed air has more mass in small volume.

What would you do for A/C when you're sitting on the ground?

The problem is pressure. In order to use outside air for the cabin, it needs to be pressurized. Otherwise you could not reach higher pressures inside the cabin than outside.
The advantage of using bleed air is that this air is already pressurized, so you don't need to do any additional work. It is however too hot to be used as cabin air directly, so a PACK (Pneumatic Air Cycle Kit) is used to reach lower temperatures using ram air for cooling.
Ram air cannot be directly used without increasing its pressure. On the Boeing 787, which is equipped with bleed-less engines, the outside air therefore needs to be pressurized with electrically driven compressors:

In the no-bleed architecture, electrically driven
compressors provide the cabin pressurization
function, with fresh air brought onboard via
dedicated cabin air inlets. This approach is
significantly more efficient than the traditional
bleed system because it avoids excessive energy
extraction from engines with the associated
energy waste by pre-coolers and modulating
valves. There is no need to regulate down the
supplied compressed air. Instead, the compressed
air is produced by adjustable speed motor compressors at the required pressure without significant
energy waste. That results in significant improvements in engine fuel consumption.

(Boeing Aero Quarterly - 787 No-Bleed Systems, emphasis mine)
Note however that this compression also adds heat to the air, so the air still needs to go through a PACK before it can be used as cabin air:

In the 787 electrical architecture, the output of the
cabin pressurization compressors flows through
low-pressure air-conditioning packs for improved
efficiency. The adjustable speed feature of electrical motors will allow further optimization of
airplane energy usage by not requiring excessive
energy from the supplied compressed air and
later regulating it down through modulating valves
resulting in energy loss.
Avoiding the energy waste associated with
down regulation results in improvements in engine
fuel consumption, and the environmental-control
system air inflow can be adjusted in accordance
with the number of airplane occupants to achieve
the lowest energy waste while meeting the air-flow
requirements.

(Boeing Aero Quarterly - 787 No-Bleed Systems, emphasis mine)
You can see the inlet for the compressors and the ram air inlet in the following picture:

(image source)

An interesting follow up question: Is the CAC Deflector Door ever opened in flight, or only when it's on the ground getting AC/Heat from ground support?

@FreeMan To be honest, I don't know too much about the 787 and I don't have access to an FCOM. This Facebook post (where I got the image from) says: "Ahead of the CAC inlet is its deflector door. It is there to prevent FOD from entering the CAC during landing and ground operations but when the outside temperature is too high (about 95°F ) or there is a fault, the deflector door will close."

Well, I wasn't really expecting an answer (I know better than to really ask a new question in the comments :D ), but that is interesting. FOD deflection makes sense since it's a pretty low inlet. Makes you wonder why they didn't use a ram air intake (or NACA duct) like the opening above. (Really, who doesn't love a good NACA duct?)

Thermal coefficient of air is around 1kJ/(kg)(K).  So KE at 400mph is sufficient to change temperature of the same mass by around 16K.

Thanks! I have been reading about "air conditioning packs" for years without suspecting that PACK is actually an acronym. TIL...


text:
What's wrong in this engine test cell picture from the book 'Ignition'?
This is picture taken from 'Ignition' book by John D Clarke.

I am bewildered after seeing picture that I can't find wrong in this picture. I guess this is related to the structure of mach diamonds.

For those not familiar with this book, it's highly entertaining, very informative, and provide excellent insights into some of the engineering and chemistry challenges that make "rocket science" a cliche.

It is no jet-engine and there is no afterburner, it is a rocket engine.

It's a fascinating book. I never believed that formulating fuels and oxidizers could be so difficult...

Since the error is related to the printing edition, I'd suggest to at least include the problematic edition (paperback? digital? Kindle edition as commented?) to make it clearer.

I think there's a mix-up in the labeling of the photo.
Here's the photo and caption from a copy of "Ignition" that I just downloaded:

Here's the photo and caption from the next page:

Seen together, those two photos and captions make sense.
Somebody mixed up the images and the captions in the copy you are reading.

My 2018 third paperback printing matches this answer.

I can see a few differences in the second picture.

Just checked my Kindle edition with Amazon's online reader and it does indeed have the two photos switched around on page vi.  Good catch.

@throx If you report that to Amazon, they'll contact the author/uploader and "suggest" they correct the edit and reupload (which will result in your downloaded copy being corrected next time your device connects).

@Brian - the tree has moved from centre-right to the far left!

@BrianDrummond yes there's a car visible in the second picture that wasn't visible in the first.  Not sure why.....

@ZeissIkon and mean that no copies can be sold and the author loses their income until it is corrected - contact the author directly if possible rather than using Amazon's mechanism.

I'd suggest to at least add the earliest/known edition (2018 third paperback as commented?) of the correct printing to the answer directly to make it clearer.


text:
Vertical and horizontal separation between cruising and climbing aircraft
I was recently a passenger on a commercial aircraft at cruise altitude when I noticed another aircraft just reaching its cruising altitude while crossing our flightpath from below, at what looked like a very close distance.
I looked up the two flights (LH491, AC172 on Nov 10, 2018) on Flightradar 24, linearly interpolated the following values in Excel to roughly 2-3 seconds resolution, and calculated the great circle distance between the coordinates:

AC 172 was heading 106° at the very end of its climb to FL 350. LH 491 was heading 51°, cruising at FL 350.
The smallest horizontal separation by my calculation was about 3.67 nm (at 1300 ft vertically). When we reached 1000 ft vertical separation, that distance was about 4.7 nm.
To my understanding, aircraft at these altitudes should always have a separation of least 1000 ft or 5 nm.
Not being an aviation professional myself, could someone please help me understand what was happening here?
I'm sure there must be a duplicate question, but I couldn't find one that involves both the vertical and horizontal components.

I think you may have answered your own question note the use of the word OR

To my understanding, aircraft at these altitudes should always have a
  separation of least 1000 ft or 5 nm.

If you take a look at your data the aircraft are always separated by 1000 feet OR 5nm. The singular exception to this is when you have the separation quoted at 4.81 miles and 982ft. This can to an extent be explained away by the inaccuracies in systems like FlightRadar24

The separation of 982ft would also most likely not be "out of bounds" for the 1000ft rule, since ATC scopes only see increments of 100ft.  Per the ATC data, the two aircraft were back into the 5+ horizontal separation range before they fell out of the 1000ft vertical separation range.